As a United frequent flyer member, I get bonus points for checking in online so I always do at home, then checkin and get another boarding pass when checking my luggage at the airport. My daughter checks in on line because hubby drops them off with the luggage, she and kids check the luggage while he's parking the car and he doesn't have to get his boarding pass because she printed it from home. This has been a great saver for her, since the last two times she flew this year, Dulles airport was packed and she would've missed her flight (arriving 2 hours early) had she had to wait for hubby to arrive and get in line. And as others say, it's a good time to change your seat. When you checkin online for United, they will sometimes offer a better seat for a small fee if you're interested and they are available, which in turn opens more seats so you can change yours. But this is all on United/Ted, not sure about other airlines.